1. Field of the Invention
This invention relates to dual chamber cardiac pacemakers and pacemaker systems and, more particularly, to an implantable dual chamber pacemaker having automatic capability of determining an optimum AV interval for the patient and adjusting the pacer AV interval to the determined optimal value.
2. Description of the Prior Art
The advantages of dual chamber pacing, and more particularly pacing in different modes which are selected in response to different patient conditions, is now well recognized in the art. Early pacing systems were solely ventricular, and were sufficient for management of patient with complete heart block and Stokes-Adams attacks. However, ventricular demand pacemakers are not equipped to take advantage of atrial activity, and thus are limited in their efficiency. Subsequently, atrial synchronous, ventricular pacemakers were introduced, having a lead for sensing P signals from the atrium and another for pacing the ventricle after a suitable P-R (AV) interval. Such a pacemaker, e.g. VDI or VDD, allows the atrium to control the heart's response rate, the ventricle being paced at the atrial rate up to a predetermined upper rate limit. Such synchronous pacers have incorporated means for dealing with high atrial rates, including "block" and "Wenckebach" techniques.
Another form of A-V or dual chamber pacer that has been utilized is the sequential pacemaker (DVI), which paces both the atrium and the ventricle with an appropriate AV delay which is timed by the pacemaker. A number of commercial pacemakers have been introduced which are programmable to these and other known pacing modes. Each of the various operating modes is particularly adapted to certain circumstances that may arise in a given patient.
Since the dual sense-dual pace DDD pacemaker became commercially available, it has gained favor for the reason that it compensates for many of the disadvantages of other pacemaker modes. The classic DDD pacemaker is described in U.S. Pat. No. 4,920,965, Funke et al., in some detail. See also U.S. Pat. Nos. 4,539,991 and 4,554,921, incorporated herein by reference, which disclose other forms of DDD-type pacemakers.
More recently, the DDDR pacemaker has come to prominence. In this type of pacemaker, there is provided one or more sensors which enable the pacemaker to be rate responsive, such that the pacing interval, or escape interval, is varied as a function of one or more sensed rate-indicating parameters, rather than being fixed at a programmed value. In the DDDR pacemaker, both atrial and ventricular natural beats may occur so long as they occur prior to the respective rate responsive escape interval. See U.S. Pat. Nos. 4,467,807 and 4,951,667, which are illustrative of dual chamber rate responsive pacemakers.
There have also been disclosed multi-mode pacemaker designs having means for switching modes in response to changing patient conditions. Most dual chamber pacemakers are programmable to distinct modes, or switch automatically from one mode to another under certain prescribed conditions. See, for example, U.S. Pat. No. 4,527,568, and U.S. Pat. No. 4,920,965. However, as a general rule it is desirable to operate in a synchronized mode as much as possible, wherein an atrial sense (AS) is followed by a ventricular pace pulse (VP) which is timed to occur at an AV interval, or delay, after the AS. Likewise, if the pacemaker is operating in a mode where it is delivering both paced atrial and ventricular pace pulses, it is desired to have the VP follow the atrial pulse (AP) by an optimized AV interval.
It is known in the prior art that it is desirable to set the AV interval as a function of rate, e.g., as a function of sensed atrial rate or pacing rate. See U.S. application Ser. No. 830,656, Dual Chamber Pacemaker With AV Extension and PMT Control, filed Feb. 4, 1992 and assigned to the same assignee as this application; and Baker, U.S. Pat. No. 4,856,524. Thus, at lower atrial rates (corresponding to a longer A-A interval) the AV interval is desirably longer; and at higher atrial rates (corresponding to a shorter A-A interval) the AV interval is preferably shorter. Further, it is known in the prior art that pacing parameters such as AV interval can be programmed externally.
The prior art also shows an attempt to adjust AV interval in a pacemaker as a function of a sensed variable. See U.S. Pat. No. 4,303,075, wherein AV delay is modified in accordance with a sensed measure of stroke volume. However, this disclosure deals with a fixed rate pacer, and does not suggest how AV delay can be optimzied for a rate adaptive pacer. U.S. Pat. No. 4,686,987 discloses a technique for determining stroke volume and for controlling pacing rate as a function of stroke volume, but makes no suggestion of how to adjust AV delay in a rate adaptive pacer. There thus has remained a need for a pacing system that is capable of dynamic automatic adjustment of AV delay, i.e., a DDDR system where AV delay is adjustable through the pacing rate range.